The present invention relates to an apparatus for hemodiafiltration, and more particularly to a hemodiafilter apparatus that continuously provides sufficient fluid to the blood circulation system of a patient to maintain the patient's blood concentration at a constant level.
Patients suffering from renal failure require artificial means for removing toxic substances accumulating in their bloodstream. Typically these patients undergo hemodialysis. In hemodialysis, blood of uremic patients is detoxified through a semipermeable membrane across which lies a concentration gradient. This concentration gradient causes certain particular toxic substances accumulated in the blood to diffuse from the bloodstream and into the dialysate. One disadvantage with hemodialysis is its inability to remove particles which are larger than the filter membrane typically used for dialysis. Another disadvantage with hemodialysis is the typically low flow rate of material transport across the dialysis membrane resulting from the concentration gradient.
One known dialysis means (U.S. Patent application No. 844,238) has attempted to deal with the disadvantage of low transmembrane dialysate flow by providing a pressure gradient across the dialysis filter. The pressure gradient is accomplished by placing upstream of the dialysis filter, a volume-controlled pump, and placing downstream of it, a suction pump responsive to the transmembrane pressure of the filter. The suction pump is controlled to keep the transmembrane pressure constant. As a result, the pressure gradient adds to the transmembrane dialysate flow rate caused by the concentration gradient. This dialysis means, however, is still limited by the filter membrane used for dialysis, which is impermeable to larger molecules. In addition, as a result of the increased flow rate, this dialysis means suffers the additional disadvantage of causing a thickening of the blood of the patient. As explained later, this problem is also exhibited in hemodiafiltration, another process used to avoid the conventional disadvantages of dialysis.
In hemodiafiltration, a hemodiafilter is used which allows the passage of molecules larger than those permitted by a dialysis filter. In addition, hemodiafiltration allows a much higher convective material transport through the membrane then conventional dialysis means. In addition, unlike dialysis, hemodiafiltration may be used to pass through the filter membrane components of the blood that do not show a difference in concentration across the filter membrane. Therefore, the number of components that may be removed from the blood in hemodiafiltration is considerably higher than that in hemodialysis. Consequently, hemodiafiltration suffers from a disadvantage in causing a thickening of the patient's blood by additional loss of blood fluid. As a result, it is necessary to replace the additional fluid lost by the patient.
In a known apparatus for hemodiafiltration that deals with the above fluid loss problem (German laid open patent application No. 32 13 390), blood is reinfused to the patient to offset any fluid loss. The amount of blood to be reinfused is determined by weighing the filtrate removed from the blood, and then administering to the patient the corresponding amount of substitution fluid. This method of weighing both the filtrate and the substitution solution suffers from the disadvantages of being tedious and time consuming. In addition, the patient suffers from a temporary condition of overly thickened blood.
It is the object of the present invention to provide a hemodiafiltration apparatus that reinfuses the patient with the required amount of substitution solution, in a continuous, quick, and easy manner, to avoid blood thickening of the patient.